The embedding of tests in the Makefile works only for that is there
today - a few `--version` and `stinit -p` calls. For expanding the
test coverage, as much as it can be done for `mt`, let's use a proper
shell test runner. `shelltestrunner` is available in Debian/Ubuntu, so
it's likely to be available in other distributions too, and as such
should not be a big barrier to contribution.
